Compartmentalization of epidermal mucilage cells by new cell wall formation in Myrsine umbellata Mart. (Primulaceae).

Mucilage cells have critical ecological and functional importance, including water storage, transport, and seed protection. These cells can exhibit diverse morphologies, often accompanied by structural changes in the cell wall, which are key to their functionality. In Myrsine umbellata, epidermal mucilage cells exhibit dual secretion: phenolic compounds and mucilage. These contents are separated by a "cell wall"; however, the formation and composition of this wall after cell differentiation remain unclear. Although some studies suggest the presence of an inner cell wall, its development and chemical makeup have not been thoroughly investigated. Using immunocytochemistry and transmission electron microscopy, we found that the new wall forms continuously with the innermost layer of the outer periclinal wall. This new layer resembles primary walls in composition, containing high levels of homogalacturonans (HGs) with a high degree of methyl-esterification, and rhamnogalacturonan I (RG-I) with arabinan side chains. It also includes low concentrations of de-esterified HGs bound to calcium ions and RG-I with galactan side chains. In the mature stage, the new wall also contains higher concentrations of arabinogalactan proteins (AGPs) and cellulose. This newly formed wall compartmentalizes the mucilage cell, isolating the protoplast from the mucilaginous compartment, thus preserving the protoplast and preventing cell death in this idioblast.